1. Field of the Invention
The present invention relates to an electrophotographic apparatus and an electrophotographic method and, more particularly, to an electrophotographic apparatus provided with an optical scanner for scanning an area on a recording medium with a light beam deflected by a light reflection type deflector, such as laser beam printers, laser copiers, laser facsimile machines, and so on, and an electrophotographic method carried out by the apparatus.
2. Related Background Art
In recent years laser printers are drawing attention because of such characteristics thereof as good quality of image, quick printout, and so on. The performance of scanning the area on the recording medium while deflecting a laser beam in the laser printers considerably affects the performance of the image quality, print speed, etc. of the laser printers.
As illustrated in FIG. 1, an optical scanner is normally composed of a laser diode 100, a rotary polygon mirror 102, a light-source optical system 104 for guiding a laser beam emitted from the laser diode 100 to the rotary polygon mirror 102, and a scanning optical system 108 for guiding the laser beam deflected by the rotary polygon mirror 102 to a recording medium 106 to scan it.
Relations between the rotary polygon mirror and the incident beam are generally classified under two types described below. The first type is an optical system of a type in which the width D of the incident light beam L is narrower than the width W of one reflecting facet 112A of the rotary polygon mirror 112 and in which all the incident light beam L is guided to the scanning lens (which will be referred to hereinafter as an underfield type optical system or UFS), as illustrated in FIG. 2.
The second type is an optical system of an overexposure type in which the width D of the incident light beam L is wider than the width W of one reflecting facet 112A of the rotary polygon mirror 112 (which will be referred to hereinafter as an overfield type optical system or OFS), as illustrated in FIG. 3.
Assuming that the same scanning lens is used, the overfield optical system is characterized by lower rotational speed of a motor etc., because the diameter of the rotary polygon mirror in the overfield optical system can be lower than that in the underfield optical system. However, because the incident beam is cut down in part to be directed toward the scanning lens, amounts of emerging light differ depending upon positions on the recording medium, which poses a problem of the locally different densities of image or the like.
To overcome the problem, U.S. Pat. No. 3,558,208 suggests that a light amount distribution of the light beam L is preliminarily shaped by a transmittance-distributed filter 126 before incidence of the incident beam to the rotary polygon mirror 112, as illustrated in FIG. 4, so as to make light amounts of emerging rays toward the scanning lens 128 constant. This device, however, had the problems of the increased number of parts, increased cost, etc. because of the use of the special filter.
On the other hand, it is known that inconsistency of the light beam reflected by one reflecting facet 112A of the rotary polygon mirror 112 can be reduced by increasing the width of the light beam L incident to the rotary polygon mirror 112 by the light-source optical system, as illustrated in FIG. 5.
Further, image forming methods of the electrophotographic apparatus of the digital type are generally classified under two types described below, as to relations between image information and exposed part. The first type is an image exposure method for exposing image areas to light (hereinafter referred to as IAE) and the second type is a background exposure method for exposing nonimage areas (background areas) to light (hereinafter referred to as BAE).
BAE is the same image forming method as that used in the electrophotographic apparatus of the analog type and thus has the merit of permitting component sharing of development, cleaning, developer, etc. with the electrophotographic apparatus of the analog type.
On the other hand, IAE requires inversion development with a developer of opposite polarity in order to gain a normal image.
Both types are put in practical use, and in many cases either one of them is determined according to restrictions such as a photosensitive member, the developer, etc. used.
A variety of developing methods, including one-component development, two-component brush development, and so on, are devised or adopted according to the needs, such as monochrome or color, or the like, and it is understood in general that image reproduction characteristics in the two-component brush development are superior to those in the one-component development. However, they have their respective features.
The principal features of the developing methods are as follows; (a) the image characteristics of the BMT method and the FEED method (one component, electrically insulative, magnetic, contact), particularly, the image characteristics of the FEED method, are approximately equivalent to those of the two component brush development; (b) the touchdown method (one component, electrically insulative, nonmagnetic, contact) has the problem of fog due to contact development; (c) the jumping method (one component, electrically insulative, magnetic, noncontact) seldom suffers the problems of fog and scratches because of noncontact; (d) the projection method (one component, electrically insulative, nonmagnetic, noncontact) rarely experiences the problems of fog and scratches because of noncontact and permits application to color image formation because of the non-magnetic property; (e) the magnedynamic method (one component, electrically conductive, magnetic, contact) involves inductive charging by a latent-image electric field and brush development to permit development of either positive or negative latent image, but transfer is difficult; (f) the IMB method (two components, electrically insulative, nonmagnetic, contact) suffers storage of charge of opposite polarity after the development because of its electrically insulative carrier, and reproducibility is not good in solid areas but is good in fine lines; (g) the CMB method (two components, electrically conductive, nonmagnetic, contact) has good reproducibility of solid areas without storage of the opposite polarity charge after the development because of its electrically conductive carrier, but reproducibility is inferior in fine lines of low density; and so on.
In general, the development is a trade-off between fog and density and the latitude thereof considerably affects the developing performance. The latitude of BAE is wider than that of IAE.
On the other hand, the transfer-separation performance is greatly influenced by the transfer efficiency and the latitude of separation and retransfer, and the latitude of BAE is wider than that of IAE, because in IAE the potential at the nonimage areas (background areas) is higher than that at the image areas.
Since the potential of the photosensitive member is decreased at rush into the cleaning, a lot of developer is likely to attach to the photosensitive member in the cleaning section in the case of the IAE which is the developing method of lower-potential portions and, therefore, the latitude of BAE is also wider as to the cleaning than that of IAE.
As described above, BAE has the potential of easier design and, as a result, the potential of capability of supplying stable electrophotographic apparatus with wide latitude.
BAE, however, has a drawback of narrower latitude in image recording by scanning with the laser beam than IAE, as described below.
In the image recording technology by the laser beam scanning, it is generally known that the size, shape, power, etc. of the laser spot greatly affect the image quality and stability.
Particularly, in the laser beam printers based on electrophotography, a latent image is formed by exposing a uniform surface potential distribution obtained by charging to the laser beam so as to decay it, so that the shape of this laser spot affects the distribution created thereby.
In IAE, the laser beam is radiated to recording image areas (portions to become black) to decay the surface potential there and the developer is deposited onto the potential-lowered portions.
Therefore, where the laser spot diameter is large relative to the spacing of scanning lines or where the laser power is too large, the line widths of characters and lines become thicker and defaced. In BAE the laser beam is radiated to the background areas (portions not to become black) to decay the surface potential there, and the developer-deposited areas are high-potential regions where the surface potential is not decayed.
Therefore, when the laser spot diameter is large relative to the spacing of scanning lines or when the laser power is too large, the line widths of characters and lines become thinner and blurred. Therefore, there are upper limits of the laser spot diameter and power in each of the image forming methods.
In FIG. 6, the left side part shows a state of one line in IAE, i.e., a laser-on state of only one line, and the right side part shows a state of one line in BAE, i.e., a laser-off state of only one line, in which the latitude of IAE is VD-Vi, and the latitude of BAE is Vb-V2.
As apparent from this figure, in the case of BAE, when the laser spot diameter is small relative to the spacing of scanning lines or when the laser power is too small, a potential gap appears in the laser-radiated portions and V2 becomes higher, which decreases the latitude. Therefore, there are lower limits of the laser spot diameter and power against the scanning line spacing. Namely, it is known that the latitude of BAE is narrower than that of IAE.
It is thus necessary in each of the image forming methods to set the optimum laser spot diameter and power.
FIG. 7 shows the relationship among the optical energy distribution of the light beam, the photosensitivity characteristic of the photosensitive member, and the surface potential distribution on the photosensitive member. It is seen from this FIG. 7 that when the photosensitivity characteristic (EV curve) of the photosensitive member is linear, the optical energy distribution of the light beam is reflected by the surface potential distribution on the photosensitive member as it is.
The photosensitive members are roughly classified under two types, organic type and inorganic type. (Organic photoconductors (OPC)).
In recent years, various organic photoconductive materials have been developed heretofore as photoconductive materials for electrophotographic, photosensitive members and, particularly, function-separated photosensitive members in which a charge generating layer and a charge transporting layer are stacked are already put in practical use and are mounted on the copiers and laser beam printers.
It has been considered, however, that these photosensitive members normally had one significant drawback of low durability. The durability is generally classified in durability in terms of physical properties of electrophotography such as sensitivity, residual potential, chargeability, image blur, etc. and mechanical durability such as wear, scratches, etc. of the surface of the photosensitive member due to scrubbing, each of which is a significant factor to determine the lifetime of the photosensitive member.
Among them, as to the durability in terms of the physical properties of electrophotography, particularly, as to the image blur, it is known that active substances such as ozone, NOx, etc. evolving from the corona charger are the cause of deteriorating the charge transporting substance contained in the surface layer of the photosensitive member.
As to the mechanical durability, it is known that the deterioration is caused by scrubbing of the photosensitive layer in physical contact with a cleaning member such as a blade and/or a roller or the like, the toner, and so on.
In order to enhance the durability in terms of the physical properties of electrophotography, it is important to use the charge transport substance resistant to the deterioration due to the active substances such as ozone, NOx, etc. and it is known practice to select the charge transport substance having a high oxidation potential. In order to enhance the mechanical durability, it is important to decrease friction by increasing lubricity of the surface so as to resist scrubbing by paper and the cleaning member, and to enhance the releasing property of the surface so as to prevent filming fusion of toner or the like, and it is known practice to blend a lubricating material such as fluororesin powder, graphite fluoride, polyolefin base resin powder, or the like in the surface layer.
However, extreme decrease of wear posed the problem that moisture-absorbing substances produced by the active substances such as ozone, NOx, etc. were deposited on the surface of the photosensitive member and it resulted in decreasing the surface resistance and causing lateral motion of the surface charge to induce so-called image flow.
On the other hand, as to the sensitivity, the organic photosensitive members normally have electric field dependence originating in the principle of carrier conduction and the potential characteristics against exposure amount (abscissas: exposure amounts and ordinates: potentials, which will be referred to hereinafter as EV characteristics) are convex down as illustrated in FIG. 14; therefore, light amount changes in a small light amount range are readily reflected by potentials. As described, since they form the potential distribution of a bowl type as against the light amount distribution of the Gaussian distribution, they had the problem that variations in the light amount at the base of the light amount distribution greatly affected change of dot diameter, so as to affect the image quality readily.
Further, since it is necessary to wear the photosensitive member in long-term use for the above reason, the sensitivity, i.e., the EV characteristics vary in long-term use. There was thus also the problem that unsharpness of dots occurred due to optical scattering caused by change of the surface shape with wear.
In the electrophotography, the photoconductive materials for forming the photosensitive layer in the photosensitive member are required to have such properties as high sensitivity, high S/N ratio, i.e., photocurrent (Ip)/dark current (Id), possession of an absorption spectrum matched with the spectral characteristics of the electromagnetic wave radiated, quick optical response, possession of desired dark resistance, harmlessness to human bodies in use, and so on.
Particularly, in the case of the photosensitive members for image forming apparatus incorporated in the image forming apparatus used as business machines in offices, the nonpolluting property in use, as described above, is a significant factor. Hydrogenated amorphous silicon (hereinafter referred to as xe2x80x9ca-Si:Hxe2x80x9d) is a photoconductive material excellent in this point and, for example, Japanese Patent Publication No. 60-35059 describes an application thereof to the photosensitive member for image forming apparatus.
The photosensitive members for image forming apparatus using a-Si:H are fabricated normally by heating a conductive support at 50 to 400xc2x0 C. and depositing a photoconductive layer of a-Si on the support by a film forming method such as vacuum evaporation, sputtering, ion plating, thermal CVD, photo-CVD, plasma enhanced CVD, or the like. Among them, a preferred method employed in practice is the plasma enhanced CVD, which is a method for decomposing source gas by dc or high-frequency or microwave glow discharge to form an a-Si deposited film on the support.
Japanese Patent Application Laid-Open No. 54-83746 suggests a photosensitive member for image forming apparatus comprised of the conductive support and the photoconductive layer containing halogen atoms as a constituent element (hereinafter referred to as xe2x80x9ca-Si:Xxe2x80x9d). This Japanese application describes that the photoconductive layer of the photosensitive member for image forming apparatus can have high heat resistance and good electrical and optical characteristics when it is made of a-Si containing halogen atoms 1 to 40 atomic %.
Japanese Patent Application Laid-Open No. 57-11556 describes such technology that a surface layer made of a nonphotoconductive, amorphous material containing silicon atoms and carbon atoms is provided on the photoconductive layer made of an amorphous material containing silicon atoms as a matrix in order to improve the electrical, optical, and photoconductive properties such as the dark resistance, photosensitivity, optical response, etc., operating environment properties such as moisture resistance etc., and aging stability of the photoconductive member having the photoconductive layer made of the a-Si deposited film.
Further, Japanese Patent Application Laid-Open No. 60-67951 describes the technology about the photosensitive member in which a transparent, insulating, overcoat layer containing amorphous silicon, carbon, oxygen, and fluorine is overlaid, and Japanese Patent Application Laid-Open No. 62-168161 describes such technology that the surface layer is made of an amorphous material containing silicon atoms, carbon atoms, and hydrogen atoms 41 to 70 atomic % as constituent elements.
Further, Japanese Patent Application Laid-Open No. 57-158650 describes that a high-sensitivity and high-resistance photosensitive member for image forming apparatus can be obtained by making the photoconductive layer of a-Si:H containing hydrogen 10 to 40 atomic % and having absorption coefficient ratios of absorption peaks of 0.2 to 1.7 at 2100 cmxe2x88x921 and at 2000 cmxe2x88x921 in an infrared absorption spectrum.
On the other hand, Japanese Patent Application Laid-Open No. 60-95551 discloses the technology for preventing the decrease of surface resistance due to adsorption of water at the surface of the photosensitive member and the image flow occurring therewith by carrying out the image forming steps of charging, exposure, development, and transfer while keeping the temperature near the surface of the photosensitive member at 30 to 40xc2x0 C., in order to improve the image quality of the amorphous silicon photosensitive member.
These technologies have improved the electrical, optical, and photoconductive properties and the operating environment properties of the photosensitive members for image forming apparatus, and the image quality has been also improved therewith.
The emission distribution of the laser is an approximate Gaussian distribution in one dot and in UFS all the laser beam reaches the drum surface as it is. Therefore, the potential was apt to reflect the light amount change in the small light amount region in OPC having the EV shape convex down as described above, so that UFS had the problem that with variations in light amounts at the base of the Gaussian distribution the dot diameters fluctuated, so as to affect the image quality.
Particularly, as the dot diameters become smaller and smaller with progress in enhancement of image quality, the effect of the dot diameter fluctuations on the image quality becomes significant, to constitute hindrance to the enhancement of image quality.
Further, it is preferable to employ BAE having higher latitude and higher stability, for designing the electrophotographic apparatus, but BAE requires setting of the optimum laser spot diameter and power at higher accuracy than IAE, because there are the upper and lower limits of the laser spot diameter and power against the scanning line spacing as described above.
There was, however, a limit of high-accuracy setting in the assembling step of the practical electrophotographic apparatus and there sometimes occurred the dot diameter fluctuations due to variations in laser emission, depending upon the setting conditions.
An object of the present invention is to provide an excellent electrophotographic apparatus whose dot diameter and image quality are less affected with light amount change at the base of the Gaussian distribution when the electrophotographic apparatus is constructed using the overfield type optical system and whose setting latitude is wide, and also provide an electrophotographic method carried out in the apparatus.
The above object is accomplished by the electrophotographic apparatus and electrophotographic method according to the present invention. The present invention can be summarized as follows; the electrophotographic apparatus using the overfield type optical system and a-Si having the linear EV characteristics, wherein the surface of the rotary polygon mirror is set at a position where the spot light amount distribution is expanded, thereby using only a portion of relatively stable light amounts near the center of the spot light amount distribution.
Specifically, the electrophotographic apparatus is provided with an optical scanning device for guiding a light beam onto a rotary polygon mirror and deflecting the incident light beam and the electrophotographic apparatus is characterized by using an a-Si base photosensitive member and satisfying the relation of 3.0xe2x89xa7D/Wxe2x89xa71.5, where D is a diameter of the light beam at a reflecting surface of the rotary polygon mirror and W is a width in a main scanning direction of a deflecting surface of the rotary polygon mirror.
Further, the electrophotographic method is carried out with an optical scanning device for guiding a light beam onto a rotary polygon mirror and deflecting the incident light beam and the electrophotographic method is characterized by using an a-Si base photosensitive member and satisfying the relation of 3.0xe2x89xa7D/Wxe2x89xa71.5, where D is a diameter of the light beam at a reflecting surface of the rotary polygon mirror and W is a width in a main scanning direction of a deflecting surface of the rotary polygon mirror.
The above described structure makes it possible to form a stable dot latent image with little potential fluctuations (fluctuations of dot diameter) even if there are variations in light amounts at the base of the light amount distribution. Further, use of OFS make it possible to make compact the rotary polygon mirror. Therefore, the apparatus can operate at high speed and can achieve very good matching with a-Si having the high durability. Further, since in BAE the affect of instable dot latent image on the image quality is more prominent, the affect of the present invention can be demonstrated better in BAE.
On the other hand, since angles of the light beam incident to the surface of the photosensitive member differ because of the principle of scanning of the light beam by the rotary polygon mirror, there appears a light amount distribution (field angle characteristics) in the axial direction on the surface of the photosensitive member. However, the field angle characteristics can be controlled by the thickness and/or quality of film by use of the a-Si photosensitive member having a coherent surface layer, because interference changes the amount of incident light.